1. Field of the Invention
The present invention relates to a method of, and a system for starting an engine in a vehicle mounted with a secondary battery.
2. Description of the Related Art
A vehicle today may be mounted with a lithium secondary battery which is constituted as a battery pack achieved by, for instance, connecting in series a plurality of lithium ion cells. If a drive source of this vehicle is an internal combustion engine, the lithium secondary battery is utilized as the power source when driving the starter motor to start the engine.
The output performance (discharge characteristics) of a battery is greatly affected by the temperature under normal circumstances. It is difficult to achieve a sufficient output power level with a lithium secondary battery at a low temperature of, for instance, xe2x88x9230xc2x0 C. In such a case, the torque and the rotation rate required at the starter motor cannot be assured and thus, the engine cannot be started.
Japanese Laid-Open Patent Publication No. H 10-285813 discloses a technology through which the required power level is obtained by heating the lithium secondary battery when the temperature is low.
FIG. 6 shows a schematic structure that may be assumed when this prior art technology is adopted in an engine starting system. A starter motor 30 is connected to a lithium secondary battery 1 via a starter relay 6 connected to a key switch 2. This engine starting system includes the lithium secondary battery 1, a heater 32 that heats the secondary battery 1 from the outside, a temperature sensor 34 that detects the temperature in the vicinity of the secondary battery 1, a voltage sensor 9 that detects the terminal voltage at the secondary battery 1 and a battery controller 36 that controls the overall operation. The battery controller 36 is supplied with power from the secondary battery 1 via an ignition relay 3. The heater 32 is controlled via a heater switch 33.
If the terminal voltage at the lithium secondary battery 1 is equal to or lower than a specific voltage level and the temperature in the vicinity of the secondary battery 1 is low when the key switch is turned on and the starter motor drive starts, the battery controller 36 engages the heater 32 in operation to heat the vicinity of the secondary battery 1. Once temperature of the secondary battery 1 reaches a level at which appropriate discharge characteristics are achieved and the terminal voltage at the secondary battery 1 exceeds the predetermined voltage level, the battery controller 36 implements control on the starter relay 6 so as to supply the power from the lithium secondary battery 1 to the starter motor 30.
However, the engine starting system adopting the prior art technology described above requires the heater 32 for heating the lithium secondary battery 1 from the outside and the temperature sensor 34 for detecting the temperature in the vicinity of the secondary battery 1 in addition to the voltage sensor 9, and thus, the number of required parts increases. In addition, since the battery controller 36 must engage in complex control processing during which the heater 32 is engaged in operation depending upon the terminal voltage at the secondary battery 1 and the temperature in the vicinity of the secondary battery 1 and then the starter relay 6 is controlled based upon the terminal voltage at the secondary battery 1, the cost of the system is bound to be high. Furthermore, since numerous sensors must be utilized to start the engine, the reliability of engine start cannot be assured easily. For instance, if a failure occurs at the temperature sensor 34, the engine start control is disabled.
It is possible to reduce the production costs by not providing the voltage sensor 9. Namely, the lithium secondary battery 1 may be heated from the outside by the heater 32 and it may be judged as to whether or not power can be supplied based upon the temperature in the vicinity of the secondary battery 1. However, since the secondary battery 1 is heated from the outside in this method, there is bound to be a difference between the surface temperature of the secondary battery 1 detected by the temperature sensor 34 and the temperature inside the battery 1. Thus, a problem arises in that very different output characteristics may manifest even when the temperature in the vicinity of the secondary battery 1 detected by the temperature sensor 34 remains unchanged. The difference between the surface temperature and the internal temperature becomes more pronounced when the temperature of the secondary battery 1 is low.
This problem is now discussed in further detail in reference to FIG. 7. Voltage characteristics D indicate the change in the terminal voltage that occurs over time after the heater 32 starts to heat the secondary battery 1. Temperature characteristics H1 indicate the change in the temperature near the secondary battery 1 and temperature characteristics H2 indicate the change in the internal temperature of the secondary battery 1. The discharge characteristics that allow the engine to be started by the starter motor 30 are achieved at the temperature xcex4. Namely, the engine cannot be started at the temperature lower than the temperature xcex4.
When the control described above is implemented, the heater 32 first heats the vicinity of the secondary battery 1 if the temperature of the secondary battery 1 is low. Subsequently, power is supplied to the starter motor 30 after the temperature (H1) detected in the vicinity of the secondary battery 1 reaches the temperature xcex4 (point P1). However, since the internal temperature (H2) of the secondary battery is lower than the temperature xcex4 in reality, the heater 32 must be further driven over a considerable length of time. Since the power from the secondary battery 1 is utilized to drive the heater 32, the terminal voltage (D) continues to decrease. As a result, the terminal voltage is lowered to a lowest allowable voltage C (point P2) without achieving a sufficient output for starting the engine, and thus, the engine cannot be started. Under these circumstances, internal shorting may occur in the secondary battery itself to reduce the service life, as well.
It is also possible that a reference value indicating the temperature level at which it is judged that power can be supplied to the starter motor 30 may be set higher than the temperature xcex4 by taking into consideration the difference between the surface temperature and the internal temperature at the secondary battery 1. However, this method poses a problem in that as the heater 32 is supplied with more power than it requires from the secondary battery 1, the available power in the secondary battery 1 is wasted.
An object of the present invention is to provide a vehicle engine starting method and a vehicle engine starting system that make it possible to start an engine with a high degree of efficiency at a low temperature, prevent wasteful power consumption and achieve a high degree of reliability.
In a vehicle engine starting method for driving a starter motor by using a secondary battery as a power source, a voltage at the secondary battery is detected, the detected voltage at the secondary battery is compared with a predetermined value and a power discharge from the secondary battery to the starter motor is allowed if the detected voltage of the secondary battery is equal to or higher than the predetermined value and a discharge is disallowed from the secondary battery to the starter motor if the detected voltage is lower than the predetermined value.
A vehicle engine starting method for supplying power in a secondary battery to a starter motor provided to start an engine via a key switch and a starter relay comprises an engine start procedure during which a voltage at the secondary battery is detected, the detected voltage at the secondary battery is compared with a predetermined value, the starter relay is set in advance in an on-enabled state if the detected voltage at the secondary battery is equal to or higher than the predetermined value whereas the starter relay is set in advance in an on-disabled state if the detected voltage is lower than the predetermined value and the starter relay is turned on to discharge power from the secondary battery to the starter motor as the key switch is set to a start position while the starter relay is in the on-enabled state, and the engine start procedure is repeated until the engine is started.
A vehicle engine starting method for supplying power in a secondary battery to a starter motor utilized to start an engine via a key switch and a starter relay comprises an engine start procedure during which a detection is made to determine whether or not the key switch is set at a start position, a voltage at the secondary battery is detected once the key switch is set to the start position, the detected voltage at the secondary battery is compared with a predetermined value and the starter relay is turned on to discharge power from the secondary battery to the starter motor if the detected voltage at the secondary battery is equal to or higher than the predetermined value whereas the starter relay is turned off to disallow a discharge from the secondary battery to the starter motor if the detected voltage is lower than the predetermined value, and the engine start procedure is repeated until the engine is started.
The vehicle engine starting system that drives a starter motor to start an engine by using a secondary battery as a power source comprises a voltage sensor that detects a voltage at the secondary battery and a control device that implements control so as to allow power from the secondary battery to be discharged to the starter motor if the voltage at the secondary battery detected by the voltage sensor is equal to or higher than a predetermined value and disallow a discharge from the secondary battery to the starter motor if the voltage at the secondary battery detected by the voltage sensor is lower than the predetermined value.